As is known, a mechanical contact, in particular in a watch, having an open position and a closed position, cannot switch from one position to the other without bouncing. Such bouncing of course adversely affects the signal produced by the contact.
The present invention relates to a circuit able to supply, in response to the signal issued by a contact troubled by bouncing, a well defined signal that is representative of the position of this contact.
Such circuits, also known as debouncing circuits, are well known. One such circuit comprises a one-shot flip-flop whose output issues a signal of given duration in response to the first closure (or first opening) of the contact. As a result, the contact's bounces immediately after the issuance of the signal no longer have any effect on the signal if the duration of the latter is greater than the switching time of the contact. This switching time varies inversely to the contact's operating frequency, i.e. the frequency of its changes of position.
This type of circuit is satisfactory in the more usual applications, i.e. when it is associated with a contact having a substantially constant switching time not affected by outside impacts. In other applications, such a circuit is liable to provide erroneous data.
For example, if the contact's switching time varies and exceeds the duration of the signal supplied by the circuit, the bouncing will still generate one or more undesirable signals, suppying false data. Such parasitic signals will also be generated by the circuit if the contact moves from one position to the other, while at rest, as a result, for instance, of accidental impacts of sufficient intensity. A further example may be when the contact's operating frequency is sufficiently high for the circuit's output signal to spread over several changes of position of the contact without detecting them. In such a case the circuit will also supply incorrect information.
Now, such extreme working conditions may, in particular, apply to certain contacts used in watches. This is for instance the case with analog electronic watches having a peripheral calendar and whose circuit receives a daily signal produced by a contact. This contact, which generally consists of a pair of flexible blades, is actuated once a day by a cam arranged on the drive shaft for the hours hand. In normal operation, the contact is closed by the cam once a day, the switching time between the instant when the contact blades start touching each other and the instant when they touch each other firmly being something of the order of 30 minutes. In the correction mode, however, the contact works far more frequently as it is actuated manually via the watch's crown, the latter then being kinematically linked to the drive shaft for the hours hand. The contact may, in this case, be actuated several times a minute, the interacting time between the blades being then reduced to a few seconds. Clearly, a one-shot flip-flop circuit cannot be used when the operating conditions vary within such limits.
Also, to save space and reduce the torque on the cam, it is preferred to use flexible blades that travel only over a short distance. This, of course, renders the contact particularly sensitive to impacts, against which a one-shot flip-flop is in no way protected.
In another constructional form, that is less sensitive to disturbances caused by impacts and which is described in Swiss Patent Application No. 4130/74, the debounce circuit essentially comprises a counter, able to count up to N, which issues, once filled, a signal at its output, and a generator which supplies a fixed frequency signal made up of a succession of pulses. When the contact is closed the pulses are applied to the counter's input, whereas while the contact is open the pulse flow is interrupted and the counter is reset.
Under these conditions and upon the contact switching from the open position to the closed position, the counter is reset whenever the contact opens because of bouncing or as a result of impacts. The pulse repetition frequency and the number N are furthermore so selected that during this period of instability on the part of the contact, the counter may not receive N consecutive pulses. Only after the final bounce, when the contact is closed with sufficient pressure for it no longer to be sensitive to shocks, will the counter, after having counted N pulses, issue a signal that is representative of the closed position of the contact.
Although the second circuit provides data about the contact's position that is more reliable than the first when the contact's operating frequency is fixed, both circuits suffer from the same deficiencies when this frequency varies to a large extent.
An object of the invention is to overcome this drawback by providing a circuit for shaping the signal produced by a contact, such signal being in either of first and second states, one state corresponding to the open position of the contact and the other state corresponding to the closed position thereof, said contact being liable to bounce as it switches from one position to the other, said circuit comprising:
means for producing a reference signal made up of a succession of pulses, the repitition frequency of the pulses being greater than the frequency with which the contact's position changes; PA0 means for counting the pulses; PA0 means for causing the pulses to be applied to the counting means when the signal from the contact is in its first state, and for causing such application of the pulses to be interrupted when the signal is in its second state; PA0 means for resetting the counting means when the signal from the contact is in its second state; and PA0 means for producing an output signal when the number of pulses counted by said counting means reaches a predetermined value, wherein the means for producing the referece signal include means for rendering the repitition frequency of this signal's pulses representative of the frequency with which the contact's position changes.